Giant Landslides: Kilauea and Mauna Loa

GEOL 205: Lecture Notes

Movement of Kilauea and Mauna Loa

While there is no real consensus on why Hawaiian volcanoes move, the fact is that they do move. One of two things can happen when you apply enough force to move a volcano; neither is particulary good.
The Hilina Slump
First, the volcano can move or slide fairly easily along its base. Generally the movement is relatively continuous, however a bit of sticking here and there can generate some rather large earthquakes. The structure that results from this type of movement is called a slump. Generally, a large block of the volcano slides coherently and stretches the volcano. Because the block is lengthening, it must also get thinner. The result of this is a set of cuspate normal faults near the head of the slump. The blocks on the seaward side of the fault drop downward due to the fact that the moving block is thinner.

The Hilina slide on the southern side of Kilauea is an excellent example of a slump. The large "palis" or cliffs on the south side of Kilauea are the tops of the extensive fault system at the head of the Hilina slide. The faults downdrop blocks towards the coast over 2000 feet in places. The downdropped coastal side of the fault blocks are probably tilted back towards the rift zone, but have been filled in by numerous lava flows making them slope very gently (about 1 degree) towards the ocean.

During the 1990's, Kilauea was moving an average of about 10 cm a year seaward. This is a lot of movement for such a large object. In contrast, Mauna Loa doesn't seem to have moved much at all in this timeframe. One possible reason for this is that Kilauea is in the way of Mauna Loa to the south and may be acting as a "door stop". Occasionally Mauna Loa does appear to move to the south, but when it does it has to essentially shove Kilauea as well. This is a lot of ground to break and as you can imagine it results in some pretty large earthquakes. The last time that this appears to have happened was in 1868 and the result was the M8 Kau Earthquake. During this event the entire coastline from Kau to Kalapana was rocked by large earthquakes and probably moved seaward. Another large earthquake struck Kalapana in 1975. This time only the south flank of Kilauea moved, but the result was a magnitude 7.2 earthquake that also dropped the southern side of the island 2-3 m in places. The style of more recent earthquakes suggests that much of this movement is taking place at the volcano/seafloor boundary or along parallel zones of weakness inside of the volcano. Seismic studies show that the seafloor is bent down only a few degrees beneath the south side of Kilauea, making it fairly easy for the volcano to slide.

A system of older faults, similar to the Hilina Pali faults, exists on the southeast side of Mauna Loa. These apparently formed before Kilauea became large enough to impede the seaward movement of Mauna Loa. However, the lowest and most northern of these fault systems, the Kaoiki fault, remains very active today. The most recent earthquake along the Kaoiki fault was a M 6.3 in 1983 that caused a great deal of damage in the Volcano Golf Course subdivision, and some damage in Hilo. Those nice "Fault Crossing" signs on Highway 11 just past the Park entrance are there to warn you that the Kaoiki fault system sometimes moves the road up and down a few feet (kinda hard on the tires if you hit one of these offsets!).

The West Side Story

The west side of Mauna Loa tells a very different story than the south flank of Kilauea. The southwest quarter of Mauna Loa faces directly out to the ocean like the south side of Kilauea. However, the large palis that are so prominent on Kilauea are absent on this side of Mauna Loa. Instead this side of Mauna Loa is characterized by unusually steep slopes with a pair of arcuate cliffs at either end of this zone (Kealakekua Bay at the north end and the Kahuku scarp on the west side of South point at the south end). These two scarps differ from the Hilina pali scarps in that their height increases towards the ocean, while the Hilina scarps generally decrease in size towards the flat coastal plain.

Sonar surveys conducted by Pete Lipman and Jim Moore of the USGS in the late 1980's off the west side of Mauna Loa identified some very surprising deposits. Many parts of the submarine flank of Mauna Loa were found to drop off precipitiously into the ocean and the ocean floor at the base of these steep scarps were littered with blocks of rock that are 2-3 times the size of Kilauea caldera. These deposits were found to distances of more than 50 miles away from the island. If you were standing on the coast and looking out to see, the ends of the deposits would be beyond the horizon. In fact, the scientists studying these deposits could barely see the summit of Mauna Loa from the ship they were using to map the deposits. Dredge samples of the blocks and sampling by deep submersible (mini sub) a few years later confirmed that these rocks had compositions identical to Mauna Loa and were not old seamounts (which would have MORB compositions).

The alternative to simple sliding (like the south side of Kilauea) is complete failure of the flank of the volcano. If the volcano is unable to slide, the pressure builds up and begins to deform or bulge out the side of the volcano. Eventually the flank of the volcano will fail catastrophically. Such failures result in debris avalanches, which are mixtures of large blocks, pulverized rock, and water. The failure of the north side of Mt. St. Helens produced a very large debris avalanche deposit. Hawaiian debris avalanches are emplaced below sea level and contain a tremendous amount of water, which makes them very fluid and capable of travelling great distances.

The west side of Mauna Loa has no apparent impediment to movement, yet still it doesn't seem to be moving seaward (in this case westward) either. Both the forcible intrusion and the deep cumulate model predict that this side of Mauna Loa should be mobile. So are the processes governing Mauna Loa completely different than those controlling the motion of Kilauea or is there some other difference that accounts for this difference in behavior?

One of the things to consider is the relative size and position of the two volcanoes. Kilauea is at the end of the Hawaiian Ridge and is fairly small, while Mauna Loa is extremely large and a bit farther into the ridge. We would expect the south flank of Kilauea to sit on seafloor that is just beginning to be bent under the volcano, while the seafloor beneath Mauna Loa should have been depressed much farther and presumably at a steeper angle. In fact, seismic refraction studies show that the ocean crust is bent down at least 10 degrees beneath the west side of Mauna Loa. This doesn't sound like a lot, but its really hard to push a volcano up a 10-15 degree slope!

The size of the debris avalanches and the amount of the material removed caused Pete Lipman to completely rethink a lot of the evolution of Mauna Loa. It now appears that somewhere around 100,000 to 200,000 years ago a series of massive failures took place on the southwest side of Mauna Loa. These debris avalanches took a huge "bite" out of Mauna Loa, with the two ends of the scar at Kealakekua Bay and South Point. The rough size of this scar is shown by the blue line of the main figure. The headwall scar of these avalanches actually appears to have partially intersected the active rift zone of Mauna Loa! This probably produced some really interesting eruptions, but all of the evidence for whatever might have happened is now deeply buried under the younger lavas.

This 3-d drawing by Rick Hazlett gives you a good idea for the size of the piece of Mauna Loa that was removed by the debris avalanche events. The following cross section picture from Pete Lipman's paper shows how the avalanche scar cut into the active rift zone and the effect that it had on the rift zone. The scar created a very low area along the rift that was the easiest place for lava to erupt for a long time.

This "low pressure" region appears to have caused the rift to shift westward several miles as it rebuilt in the scar (the rift zone is represented by the dikes--the vertical lines--shown on the Lipman diagram). This model is the first one to really explain why the end of Mauna Loa's southwest rift zone shows a strong offset.

An interesting effect of this was to essentially stop eruptions from reaching the southeast side of Mauna Loa for tens of thousands of years. Erosion was able to begin cutting into the large palis on the SE side of Mauna Loa. Valleys are cut when waterfalls plunge over the cliffs and begin to erode back up slope. The Ninole Hills are the eroded remnants of the large scarps that once made up the southeast side of Mauna Loa volcano (though the 3-d drawing doesn't show this scarps as being present when the SW side of Mauna Loa collapsed, the palis on the southeast side were already present as shown in the Lipman diagram). Notice that the northern end of this fault system did not suffer the same fate as the Ninole Hills. This is because lava flows from the summit region of Mauna Loa were able to continue covering this part of the fault system. During this time interval, Kilauea also grew up and is banked against Mauna Loa and has grown nearly as high as the tops of this fault system.

This map shows the landslides on both Kilauea and Mauna Loa.

This map from Moore and Chadwick (1995) shows the subaerial and submarine geology on both Kilauea and Mauna Loa.

Examination Questions

  1. Describe and explain the mechanics of the Hilina slump. Why does it behave in this manner?
  2. Describe and explain the landslides on the west side of Mauna Loa. Why do they behave the way they do?
  3. What effect did the landslides on the Southwest side of Mauna Loa have on it's magma plumbing system? Are there any surface expressions of this event today?
Extra Reading

Moore, J. G. and others, 1989, Prodigious submarine landslides on the Hawaiian Ridge: Journal of Geophysical Research, Series B 12, Volume 94, p. 17,465-17,484. Moore, J. G., and Chadwick, Jr., W. W., 1995, Offshore Geology of Mauna Loa and adjacent areas, Hawaii: in Mauna Loa Revealed, Rhodes, J. M., and Lockwood, J. P., American Geophysical Union Geophysical Monograph 92, p. 21-44. Lipman, P. W., 1995, Declining growth of Mauna Loa during the last 100,000 years: Rates of lava accumulation vs. gravitational subsidence: in Mauna Loa Revealed, Rhodes, J. M., and Lockwood, J. P., American Geophysical Union Geophysical Monograph 92, p. 45-80.

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